Diastereoselective trimolecular condensation between indole,Meldrum’s acid and chiral sugar-derived aldehydes

The trimolecular condensation of indole, Meldrum’s acid and chiral, sugar-derived aldehydes took place in good yield and high diastereoselectivity. The absolute configuration of the α-carbon of the chiral aldehydes ensured a predictable diastereocontrol of the newly created stereogenic centre except for (3aR,5S,6S,6aR)-6-benzyloxy-2,2-dimethyl-tetrahydrofuro[3,2-d][1,3]dioxole-5-carbaldehyde 3i. In this case, the opposite stereochemistry may be explained by a less congested conformer of the Knoevenagel-adduct intermediate.     

Polymer-bound osmium oxide catalysts

Polymer-supported oxidic osmium catalysts based on cross-linked poly(4-vinyl pyridine) were synthesized by various routes and characterized by a number of physical techniques (Raman, IR, XPS, ¹³C and ¹⁵N solid-state NMR spectroscopy). Model compounds of type Os₂O₆L₄ (L = pyridine, 4-iso-propyl pyridine, and 4-tert-butyl pyridine) were obtained under the conditions of the catalyst synthesis. The catalytic systems were successful in the dihydroxylation of alkenes.     

Water revealed as molecular mirror when measuring low concentrations of sugar with near infrared light

Near infrared spectroscopy is an overtone spectroscopy regarded as a quick and non-destructive method that provides analytical solutions for components that represent approximately 1% or more of the total mass of the investigated composite samples. Aquaphotomics offers the possibility for disentanglement of information remaining hidden in the spectra when conventional data evaluation methods are used, since this concept utilizes changes of the water structure induced by the measured solute as specific molecular vibrations at water bands. Here, near infrared technique and aquaphotomics are applied for non-destructive identification and quantification of mono- and di-saccharide solutes at 100–0.02 mM concentration that is accepted as unachievable with near infrared spectroscopy. The results presented in this study support the aquaphotomics' water molecular mirror concept that explores spectral changes related to water molecular rearrangements caused by minute changes of the solutes in the aqueous systems. The method provides quick and accurate alternative for classical analytical measurements of saccharides even at millimolar concentration levels.     

The geometry of the nitroguanyl fragment in the simplest nitroguanidine derivatives in the absence of intermolecular interactions: The gas electron diffraction data on 1,1,3,3-tetramethyl-2-nitroguanidine

The structures and force fields of the equilibrium forms of 2-nitroguanidine (1), 1,1,3,3-tetramethyl-2-nitroguanidine (2), and nitroguanyl azide (3) were determined in the MP2(full)/6-311G(3df, 2p) approximation; wagging-inversion motions of the N amine atoms were studied. The internal rotation potential function of the NO₂ group was calculated for 1. Similar functions for 1 and 2 were also obtained in the MP2(full)/6-311G(d, p) approximation. Direct one-dimensional problems for a nonrigid model were solved by the variational method, and the distribution of torsional levels was obtained. In the region of potential minimum, rotation in both molecules had the character of large-amplitude motions. For the first time, electron diffractions data were obtained at 100°C for molecule 2 without noticeable traces of substance decomposition. A structural r _e analysis was performed using the model of large-amplitude motions for characteristic NO₂ group torsional vibrations. Vibrational corrections to internuclear distances and mean amplitudes were calculated taking into account nonlinear kinematic effects using the force fields obtained in this work. The geometry of molecule 2 calculated in the MP2(full)/6-311G(3df, 2p) approximation well corresponds to the gas electron diffraction data. The parameters of molecule 2 in the crystalline phase, however, differ substantially from the parameters of the free molecule. This corresponds with the suggestion of the influence of intermolecular H-bonds involving the imine nitrogen atom and nitro groups oxygen atoms.     

Methanol synthesis on ZnO(0001). III. Free energy landscapes, reaction pathways, and mechanistic insights

The interplay of physical and chemical processes in the heterogeneous catalytic synthesis of methanol on the ZnO(0001) surface with oxygen vacancies is expected to give rise to a complex free energy landscape. A manifold of intermediate species and reaction pathways has been proposed over the years for the reduction of CO on this catalyst at high temperature and pressure conditions as required in the industrial process. In the present study, the underlying complex reaction network from CO to methanol is generated in the first place by using ab initio metadynamics for computational heterogeneous catalysis. After having "synthesized" the previously discussed intermediates in addition to finding novel species, mechanistic insights into this network of surface chemical reactions are obtained based on exploring the global free energy landscape, which is refined by investigating individual reaction pathways. Furthermore, the impact of homolytic adsorption and desorption of hydrogen at the required reducing gas phase conditions is probed by studying such processes using different charge states of the F-center.     

Master Equation and Fokker–Planck Equation: Comparison of Entropy and of Rate Constants

A usual approximation of the master equation is provided by the Fokker–Planck equation. For chemical systems with one species, we prove generally that the prediction of the rate constant of the metastable state given by the Master equation and the Fokker–Planck approximation differ exponentially with respect to the size of the system. We show that this is related to the fact that the entropy of the metastable state is not described correctly by the Fokker–Planck equation. We prove that the rate given by the Fokker–Planck equation overestimates that rate given by the Master equation.     

Basic modes of three-dimensional polymer crystal associations: Interlocked single crystals and crystal halves

Tridimensional associations of lamellar polymer single crystals, grown from dilute solutions, are described as derived from their sedimentation patterns. These associations include interlocked crystals and decorating crystal halves. The origin of these crystals and their mutual orientation are discussed and tentatively interpreted by specific interactions between the fold surface and the crystallizing chains.